The present invention relates to a micromirror configured to tilt a mirror in minute quantity by causing electrostatic attraction between adjacent electrodes.
Recently, various types of micro devices are in practical use with development of MEMS (Micro Electro Mechanical Systems) technology. Among such micro devices is a micromirror which can be used, for example, as a scanner adapted for a barcode reader, a laser printer, and etc. Examples of such a micromirror are disclosed in U.S. Pat. No. 6,057,952. The micromirror disclosed in U.S. Pat. No. 6,057,952 is an electrostatic driving type device configured to tilt a mirror in a minute quantity by electrostatic attraction acting between electrodes.
An example of a micromirror disclosed in U.S. Pat. No. 6,057,952 is configured such that a reflection mirror is able to tilt around two rotation axes so that two-dimensional scanning can be performed on a surface of an object. In this example, the reflection mirror is pivotably supported by a first pair of torsion bars. The first pair of torsion bars are supported by a first gimbal part formed around an outer shape of the reflection mirror, and the first gimbal part is pivotably supported by a second pair of torsion bars formed to extend in a direction which perpendicularly intersects with a direction in which the first pair of torsion bars are extended.
The second pair of torsion bars are supported by a second gimbal part formed around an outer shape of the first gimbal part. Two electrodes are formed on the reflection mirror, and two electrodes are also formed on the first gimbal part. Further, an electrode is located oppositely to the above mentioned electrodes.
When a voltage is applied between the electrode on the reflection mirror and the opposed electrode, electrostatic attraction is caused between these electrodes and thereby each of the first pair of torsion bars twists. As a result, the reflection mirror torsionally rotates about a first rotation axis. When a voltage is applied between the electrode on the first gimbal part and the opposed electrode, electrostatic attraction is caused between these electrodes and thereby each of the second pair of torsion bars twists. As a result, the reflection mirror torsionally rotates about a second rotation axis which is perpendicular to the first rotation axis. By thus applying voltages to the electrodes, it is possible to rotate the reflection mirror about two rotation axes.
By directing a beam to be incident on the reflection mirror torsionally rotating about the two rotation axes, the beam reflected by the reflection mirror swings in two dimensions. By directing the reflected beam to illuminate an objection, two-dimensional scanning can be achieved on the object.
In the micromirror disclosed U.S. Pat. No. 6,057,952, patterns are formed on the first and second pairs of torsion bars to electrically connect the electrodes on the reflection mirror to the electrodes on the second gimbal part. Patterns are also formed on the second pair of torsion bars to electrically connect the electrodes on the first gimbal part to the electrodes on the second gimbal part. The term “pattern” means a conductive pattern which is made of a thin leaf of metal (e.g. copper) and is formed on a substrate.
The micromirror is a device having a microstructure. In particular, the torsion bar is formed to have a miniscule width. Further, in general, the width of a pattern is designed considering a manufacturing error. Therefore, the width of a pattern formed on a torsion bar is designed to be narrower than the width of the torsion bar. That is, the width of a pattern on the torsion bar is extremely narrow. Since, in the micromirror in U.S. Pat. No. 6,057,952, it is necessary to form two patterns on each second torsion bar, the width of the pattern on the second torsion bar needs to be narrower than the width of the pattern on the first torsion bar.
Use of a high-precision pattern formation technology makes it possible to form fine patterns on a semiconductor substrate. However, such a high-precision pattern formation technology necessarily increases manufacturing cost of a micromirror. In addition, if a high accuracy is required for forming patterns, tolerance is decreased, which may lead to decrease of a yield of micromirrors. The decrease of a yield of micromirror may also cause decrease of the production efficiency and increase of the cost of production management.
Since a load is put on the torsion bars when the micromirror is in a driven state, if the width of a pattern is extremely narrow, the pattern formed on the torsion bar may exhibit a tendency to be easily broke during the driven state depending on material of which the pattern is made (e.g., brittle material). In other words, in order to narrow the width of a pattern to be formed on a torsion bar, a selectable range of material of the pattern is narrowed.
In addition, if the width of the pattern is narrowed, electrical resistance of the pattern increases. In this case, it is required to increase a driving voltage for driving each electrode.
If a micromirror for one-dimensional scanning is formed to have a base part which includes a torsion bar and is made of conductive material, the base part itself is able to serve as a conductive pattern. That is, in this case, formation of patterns on the micromirror is not required. Therefore, a high-precision pattern formation technology is not required. Use of the conductive base part also resolves the above mentioned problem of rupture of patterns on the torsion bar.
However, in the case of a micromirror for two-dimensional scanning, the number of signal lines to be routed to the outside of the micromirror (i.e., to a driving volage supply unit) is larger than that of the micromiorror for one-dimensional scanning. More specifically, the micromirror for two-dimensional scanning has the number of signal lines to be routed to the outside larger than the number of second pair of torsion bars (i.e., two). It is understood that, in order to configure a micromirror so that a conductive base part thereof serves as conductive patterns, the number of signal lines to be routed to the outside needs to be smaller than or equal to the number of outer torsion bars (i.e., the second pair of torsion bars in the above mentioned example of the micromirror having the two rotation axes). Therefore, it is not possible to use the design scheme of the micromirror for one-dimensional scanning to design a micromirror for two-dimensional scanning.